In the current DataGrid implementation, the DataGrid will start with all the groups expanded. The only way to have any form of preference is to programmatically interact with the DataView after data is presented in the control  https://docs.telerik.com/devtools/maui/controls/datagrid/grouping/expand-collapse 

Requested Feature
A better approach that I am requesting a feature for is to have a property available for the DataGrid that sets this value ahead of time.

For example, you could add it as a BindableProperty on the GroupDescriptor class and on the GroupDefinition is to have an IsExapanded property.